DESCRIPTION (Applicant's Abstract): What does it mean to know what a table, or a chair, or a hammer is? Despite much attention, no clear answer to this fundamental question about mental representation has thus far emerged. Progress in understanding the nature of concepts and categorization may require 1) distinguishing between two different acts of categorization, and 2) distinguishing among different processes that contribute to categorization decisions. These distinctions can be drawn within a unifying framework that specifies the relationships among concepts, categories, word sense, and reference. The investigators propose two sets of studies that will provide an empirical foundation for their framework. The studies will examine classification in a variety of experimental contexts. The first set derives directly from the distinction to be drawn between categorizing as an act of recognition and categorizing as a communicative act. These studies examine the relation between recognition, which the investigators view as based on the similarity of an object to other objects in feature space, and the names that are applied to objects. The second set of studies investigates mechanisms that produce complexity in how names are applied to objects. These studies provide additional information about the extent to which similarity can account for observed phenomena of naming, and they identify other mechanisms that may be required. Tests of a formal model embodying the investigators' assumptions about the role of similarity in producing naming phenomena are integrated with the experimental work in both sets of studies. The ultimate goal of the proposed project is to provide the empirical and theoretical foundation for a comprehensive theory of categorization.